1. Field of the Invention
The present invention relates to a power train system for a vehicle incorporating an engine and a motor. The invention also relates to a method for controlling a power train system.
2. Description of the Related Art
In recent years, an automatic engine stopping and starting apparatus (hereinafter referred to as an "economy running system") has been known for stopping an engine after a running vehicle has been stopped and restarting the engine if conditions for driving the vehicle have been satisfied again. Japanese Patent Laid-Open No. 9-39613, for example, discloses a structure of the above-mentioned apparatus in which a motor generator is connected to an engine through a power distributing mechanism. The automatic engine stopping and starting apparatus is arranged such that fuel supply to the engine is interrupted while the vehicle is stopped, resulting in reduced fuel consumption. Moreover, idle rotation of the engine is maintained by the motor generator during the operation of the automatic engine stopping and starting apparatus (hereinafter referred to an "economy running mode"). The engine is kept to have idle rotation by the motor generator while the vehicle is stopped in order to operate auxiliary machines, such as a compressor for an air conditioner and a pump for a power steering unit during such period.
The above-mentioned structure eliminates the need of electric motors to operate the auxiliary machines, such as the compressor for the air conditioner and the pump for the power steering unit. Thus, the space required for the structure of the economy running system can be minimized.
However, the above-mentioned art to operate the auxiliary machines by maintaining revolution of the engine at the idle speed cannot reduce power consumption of the motor generator for operating the engine. Since the motor generator is, in general, operated by electric power of a battery mounted on the vehicle, power consumption of the battery is increased due to frequent stop operations of the vehicle. As a result, a battery having a large storage capacity must be mounted on the vehicle.